** NEW — Now Available in AudioTools
The Transmission Loss module is available as an in-app purchase option in RTA. Also known under the names STC, NIC, OITC, and Rw, depending on the standard that is being used, the purpose of Transmission Loss measurements in any case is to determine how much acoustic attenuation a wall or divider will provide.
Types of Transmission Loss Measurements
In the USA, the Sound Transmission Class (STC) measurement is commonly used. This is define in ASTM E-336, which in turn references ASTM E-90. This measurement is computed by comparing pink noise in the source room with the noise level in the receiving room. The formula uses 1/3 octave data points from 125 to 4000Hz, and larger numbers indicate more isolation. Since this test does not include RT60 times, it is technically an NIC (Noise Isolation Class) measurement rather than an (F)STC (Field Sound Transmission Class) measurement.
Alternatively, Outdoor-Indoor Transmission Class (OITC) may be used in the USA, and is based more on an A-Weighted spectrum.
In Europe, ISO 717-1 is used, and commonly referred to as Rw. Rw is very similar to STC, but uses 1/3 octave bands from 100 to 3150Hz.
The transmission loss module in AudioTools complies with all listed standards.
How to Test for Transmission Loss
AudioTools provides both static and dynamic transmission loss test methods. Either method begins by storing the source room reference curve.
To begin, go to the RTA module in AudioTools, and tap the wrench icon to go to the Setup screen. Now, tap the Transmission Loss button to bring up the Transmission Loss setup screen. Note that if you have not yet purchases Transmission Loss, you will be presented with the options to see the demo video or see the current pricing for the module.
First, select the method: STC (US-based ASTM standard), Rw (European ISO 717-1 based measurement), or OITC (US-based A-weighted ASTM measurement)
Next, turn “Compute Sound Transmission Loss” on.
Source Room Measurement
Setup a pink noise source in the Source room. It should be as omnidirectional as possible, and should be roughly in the center of the room.
Make sure the RTA is set to 1/3 octave mode, and to a Average decay mode.
Turn on the noise source, and begin taking measurements 1 meter from the wall, and move slowly along a section of the wall, and when done pause the RTA measurement.
Store this measurement, using the Save / Recall function.
Receive Room Measurement — Dynamic
First, recall the previously stored Source Room measurement. It will show on the screen as a set of green bars.
Select a slow decay mode, turn on the noise source, start the RTA measure and measure any point along the wall, 1 meter back. You may also hold the microphone close to any suspect noise leak area, and get a qualitative measure of the amount of leakage. The Transmission Loss value will be shown in real time on the screen.
Receive Room Measurement — Static
Turn on the noise source, start the RTA measure and slowly move along the wall in the receiving room, 1 meter away from the wall. When done, pause the RTA, and save to another memory position.
To compute the Transmission loss, recall the RECEIVE room first, then recall the SOURCE room. Do not start the RTA running. The calculated transmission loss value will be displayed on the screen
ASTM E 336 specifies that if the background noise during the measurement is within 10dB of the measured signal, it must be taken into account. And, if the noise is within 5dB of the measured signal, the results can only be used as an estimate.
Since the app does not compute a noise penalization factor, you should manually confirm that the background noise during the test is at least 10 dB lower than the measured, for each octave band.